Navigation of a spacecraft is the act of determining the spacecraft's position in space. Several different techniques for navigation have been used in the past, the choice of technique varying from one spacecraft to another depending on several factors. For spacecraft equipped with propulsion capability, one of those factors is the method of propulsion used.
Many different technologies are used for spacecraft propulsion. High thrust propulsion, typically provided by chemical rockets, is required by launch vehicles used to launch spacecraft from Earth because of the high gravity near the surface of the Earth and air resistance from the atmosphere. High thrust propulsion is also used frequently by spacecraft once in space. Accordingly, navigation techniques and infrastructure have been developed that are suited to high thrust spacecraft.
One technique for navigating spacecraft—in orbit around the Earth, the Sun, or other planetary bodies—that employ high-thrust propulsion uses one or more on-board accelerometers as well as external reference navigation sensing methods such as, for example, radio tracking from Earth. Altering the trajectory of such a spacecraft can be accomplished by determining the spacecraft's initial trajectory (which can be described in terms of its position and velocity with respect to some convenient reference frame) using radio tracking prior to making a propulsive manoeuvre (a “burn”), calculating the change in velocity (Δv) required to alter its trajectory in the desired direction and amount, and performing a high thrust rocket burn while measuring acceleration (integrated to get Δv). The Δv estimate from the acceleration measurements can be used to make a decision on-board the spacecraft as to when to terminate thrusting, and a second radio tracking fix can be used to confirm the new trajectory. There is typically some uncertainty in the thrust expected to be generated by the propulsion system, because it is difficult to completely calibrate the performance of such systems in advance of use. Inertial sensing using accelerometers is used to compensate for this uncertainty by estimating the accumulation of Δv over the course of the burn, allowing an on-board decision to terminate thrusting when the desired Δv has been achieved.
However, acceleration measured by the accelerometers will include some error, and hence there will be an error in the estimated Δv. The amount of this error typically grows larger with time. Because velocity changes occur rapidly with high-thrust propulsion, the period of time over which that error can build up is relatively small (typically on the order of hours or less, and the Δv error resulting from each burn can be resolved quickly following each burn. For example, it may be corrected using one or two subsequent smaller, shorter burns, with additional radio tracking fixes performed before and after each burn.
More recently, a greater number of propulsive options have become available for propulsion in space, away from high gravity objects and drag forces. These include low thrust propulsion systems, often chosen for their greater efficiency and lower mass, such as ion engines, solar sails, Hall thrusters, VASIMIR thrusters and the like. In contrast to high thrust options, for which the desired Δv is usually accomplished within a short period of time (seconds, minutes or hours), low thrust propulsion systems can require days or weeks of continuous thrusting to achieve the Δv needed for a significant course change. Because of the prolonged thrusting, while uncertainty in the thrust generated by the propulsion system may be small, it can accumulate over time to cause large errors in the resulting trajectory. Similarly, accelerometer sensing errors and errors in the resulting value of spacecraft Δv and position can accumulate to much larger values over the duration of prolonged thrusting.
Natural effects can impart low acceleration disturbances to a spacecraft's trajectory over a prolonged period and cause that trajectory to slowly deviate from a purely ballistic trajectory. There may also be uncertainty in the magnitude and direction of the forces from such natural low-acceleration perturbation effects. For example, one such effect is solar radiation pressure. Solar radiation pressure causes a force whose magnitude and direction are sensitive to a spacecraft's orientation, to details of its shape, and to various optical properties (e.g., reflectivity) of its various surfaces, knowledge of each of which may be uncertain to some extent. Uncertainty in the small forces imparted on a spacecraft by such natural effects can cause uncertainty in the spacecraft's trajectory to accumulate to large values over long periods of time. These periods of time can be long enough that accumulating errors from accelerometers render inertial sensing of such changes in the trajectory impractical. As a result, external reference navigation sensing techniques, such as radio tracking, relative navigation using on-board optical sensors, and other techniques such as ranging using laser communications have been preferred in low-thrust missions. However, these methods require communication with a ground station on Earth to make a navigational “fix.” Over months or years of low-thrust manoeuvring, many such communications sessions may be needed to maintain an accurate assessment of the evolving spacecraft trajectory.
Radio tracking and communications with spacecraft is expensive, particularly for spacecraft far from Earth, and existing facilities, such as NASA's Deep Space Network (DSN), are already oversubscribed. Several current trends are likely to make this situation worse. The cost of access to space is declining, low thrust propulsion options are becoming more popular and spacecraft are getting smaller and more affordable leading to a greater number of spacecraft being built. Consequently, the number of spacecraft needing to be tracked and the total tracking time required seem set to increase rapidly, making it harder for the existing tracking infrastructure to keep up with the growing demand.